Reference is made to the U.S. Pat. Nos. to Roeder 3,703,926; Roeder 3,650,640; Roeder 3,540,814; and Coberly 3,322,069 for further examples of the prior art.
Hydraulically actuated downhole pump assemblies are known to those skilled in the art as evidenced by the above referred to patents, and to the art cited therein, to which reference is made for further background of this invention.
In drilling boreholes to depths exceeding 15,000 feet, it is necessary to reduce the diameter thereof for obvious reasons. Accordingly, when it becomes necessary to utilize a downhole fluid actuated pump assembly in a slim or narrow borehole, it is desirable that the pump assembly have a minimum cross-sectional area so that the pistons contained therein can be fabricated to have a maximum cross-sectional area, thereby pumping a proportionately greater volume of production fluid for each stroke of the pump.
In U.S. Pat. No. 3,650,640, there is taught a pump having a longitudinally extending hollow body member with the interior thereof being divided into upper and lower chambers by a valve assembly. The pump assembly has opposed pistons, one in each chamber, dividing each of the chambers into upper and lower chambers, thereby enabling the valve assembly to be placed within the very closest proximity of the engine cylinder or piston chambers.
In the above copending patent, the use of external passageways formed either through the main body of the pump assembly or the placement of flow conduits externally of the body has been avoided. Both of these expedients significantly reduce the effective cross-sectional area of the engine and the pump pistons. However, it is sometimes desirable to use a downhole pump in a situation which tolerates the presence of external passageways of this type, and such a desirable contribution is the subject of this invention.